That's exactly why more are trying tricks to check if we have read the whole ad. That might take the form of putting "peanut butter - crunchy" in the subject line.
Also, some explicitly state that if we don't follow every step of how they want us to compose the cover letter, they will toss it. They mean it.
So, if they request 1 brief writing sample, only put the link to 1. And that 1 should be short.
Being able to follow their directions is, in a sense, a mini interview. Employers have to ensure that you will be able to carry out the policies and procedures they have on the job itself. They figure: Hey, if this applicant, can't even get the cover letter the way we want it, that person will screw up everything here.
So, take time to read the whole ad. Follow directions with the same care that you fill out government forms.